All cells are exposed to physical and chemical environmental agents. The integrity of their genetic information is largely maintained by enzymatic repair processes. The cellular slime mold, Dictyostelium discoideum, serves as a useful model system for studying repair in eukaryotic cells. Radiation and chemically sensitive and resistant strains are available. Repair can be studied in the actively replicating vegetative cells or in the non-replicating developing cells during differentiation to spore and stalk cells. These experiments will study: (1) the stimulation by radiation of unscheduled nuclear and mitochondrial DNA synthesis during development and its relation to repair processes; (2) the effects of radiation on RNA synthesis at various stages of development; (3) the genetics of radiation and chemical sensitivity and resistance in several mutant strains previously isolated here; (4) the enzymatic defects of DNA repair in these mutants; and (5) the metabolism of DNA precursors in semiconservative and repair replication, with emphasis on thymine and its derivatives. BIBLIOGRAPHIC REFERENCES: Deering, R. A. and D. S. Jensen "Endonuclease from Dictyostelium discoideum that attacks ultraviolet-irradiated deoxyribonucleic acid" J. Bacteriol. 121, 1211-1213 (1975). Deering, R. A. "Dictyostelium discoideum: A valuable eukaryotic system for repair studies" in Molecular Mechanisms for Repair of DNA, Part B (ed. by P. C. Hanawalt and R. B. Setlow) Plenum, 1975.